Field of the disclosure
This disclosure relates to fiber lasers and, more particularly, to a multimode fiber Fabry-Perot fiber laser having a resonant cavity which defined between multimode fiber gratings and capable of emitting multimode light in a narrow spectral line.
Disclosure Prior Art Discussion
Traditionally, fiber oscillators are configured with single mode (“SM”) active and passive fibers or other mode filter components providing generation of substantially a single transverse mode. One of ordinary skilled in the laser art is well aware of advantages offered by SM fiber oscillators. Yet SM fiber lasers also face a few challenges some of which are discussed immediately below.
As known, there are limitations imposed on the geometry of SM active step-index fibers. The SM core cannot be limitlessly enlarged since it practically would not be able to support only a SM upon reaching about a 30 micron core diameter. Yet larger core diameters are desirable for generating of high power SM laser beams. The primary cause for such a limitation includes various non-linear effects (“NLEs”) which, despite concerned efforts on part of the scientific and manufacturing communities, remain a problem. In particular, NLEs are damaging for high power fiber lasers and amplifiers.
As well known to one of ordinary skill in the fiber laser art, NLEs are one of the major factors liming the power scaling of fiber lasers. Different NLEs have the following in common:
A. The pulse optical power at which they set on (referred to as “threshold power” and coinciding with the maximum pulse power achievable) is proportional to the fiber core area and inversely proportional to the fiber length. In other words, long fibers of small core used in fiber lasers favor NLEs; and
B. NLEs cause unwanted spectral broadening of the laser emission spectrum and/or wavelength conversion, and optical feedback, power instabilities and ensuing potential damages to the optical components. In particular, the unwanted broadening observed in response to the increased power density is a result of four wave mixing (“FWM”) NLE. Also, a Raman NLE is manifested practically by detrimentally affecting affection the operation of fiber devices.
The undesirable formation of long-period fiber gratings (“LPFGs) may also be disadvantageous. In this type of structure, the signal light interacting with the LPFG is coupled into cladding modes where it rapidly attenuates due to absorption and scattering. Accordingly, the signal may experience a loss which, regardless of how low it can be, is clearly undesirable.
An unstable laser operation may be observed due to the excitation of high order modes, if a gain medium includes low mode active fibers. Typically, a SM laser is configured with the low mode active fiber having its opposite ends fused to respective SM passive fibers. If mode field diameters of SM and fundamental mode of respective passive and active fibers are not matched, the excitation of high order modes is difficult to suppress. However, such low-mode active fibers are the choice of design in practically all high power lasers/amplifiers configured to emit a beam in substantially a single, fundamental mode. As a consequence additional manufacturing efforts should be applied to have the MFDs of respective SM and MM fibers substantially match.
For SM lasers with low-mode active fibers, the output beam has a close to Gaussian intensity profile characterized by a pointed top. Such a beam is not always ideal for certain applications. Frequently, a uniform intensity distribution associated with the profile's flat top is advantageous. In this case, SM lasers should be additionally configured with beam shaping optics which increases the overall cost of the systems.
A need therefore exists for a MM fiber oscillator having a configuration successfully meeting the above-disclosed challenges.